Motor Knock

By taking motion pictures through a quartz window in a gasoline motor's combustion chamber and by registering the 1 pressure changes, Lloyd Withrow and T. A. Boyd of General Motors were able to tell the American Chemical Society at Indianapolis last week exactly why motors knock. Quality of gasoline is the cause. With good fuel a pencil of flame darts from the spark plug and ignites all the charge progressively. This occurs in 1/250 sec. With knocking gasoline, the instant the spark starts ignition, the first burned fuel creates sufficient heat and pressure to ignite all the remaining fuel in one sharp blast, before the spark flame can do its comparatively slow duty. Antiknock compounds obviate the blast, enable the fuel to burn with proper slowness. An observation: gasoline knocks occur after ignition, not before as had been generally supposed.

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